1. Field of the Invention
The present invention relates to a light source device for use in evaluating the characteristics of an optical module such as an optical receiver or the like by intentionally superposing noise to degrade the waveform of an optical signal transmitted over an optical communication system when the extinction ratio is to be set to a random value without degrading the characteristics of the waveform of the transmitted optical signal in testing the transmission characteristics of the optical communication system.
2. Description of the Related Art
Optical modules such as receivers or the like are evaluated for their characteristics based on the cross-point level, positive-going edge, and negative-going edge of an input optical signal, the extinction ratio, and the simulation of an intersymbol interference due to an interference signal. The extinction ratio is varied by a direct modulation system using a light-emitting element such as an light-emitting diode (hereinafter referred to as “LD”) or a modulation system using an external modulator such as LiNbO3(LN), electro-absorption (EA), or the like.
FIG. 15 of the accompanying drawings shows a conventional direct modulation system. FIG. 16 of the accompanying drawings illustrates a process for varying the extinction ratio with the direct modulation system. In FIG. 16, the horizontal axis represents an LD current and the vertical axis an optical output power. As shown in FIG. 16, a light source (LD) 10 emits light when the LD current exceeds a threshold value Ith, and the optical output power increases as the LC current increases. The direct modulation system controls the optical output power of the LD 10 with the LD current supplied from a driver 12. As shown in FIG. 16, the extinction ratio is controlled by a biased value IB for the LD current which determines a low optical output power level P0 of a main signal and an amplitude IP of the LD current which determines a high optical output power level P1 of the main signal. The extinction ratio is expressed by the following equation (1):Extinction ratio=10×log(P1/P0)  (1)
FIG. 17 of the accompanying drawings shows an LN modulation system. FIG. 18 of the accompanying drawings illustrates a process of varying the extinction ratio with the LN modulation system. In FIG. 18, the horizontal axis represents a drive voltage and the vertical axis an optical output power. As shown in FIG. 17, an LN modulator 24 has its optical output power variable in certain periodic cycles as the drive voltage output from a driver 22 changes. A low input signal level is converted into a drive voltage level V0 and a high input signal level is converted into a drive voltage level V1 by the driver 22. These drive voltage levels are input to the LN modulator 24, which modulates DC light from a light source 20 with the drive voltage. Optical output power levels which correspond to the drive voltage levels V0, V1 at the time the optical output power is turned on and off are determined, thus determining an extinction ratio. In this manner, the extinction ratio is varied by controlling the drive voltage levels V0, V1.
FIG. 19 of the accompanying drawings shows an EA modulation system. FIG. 20 of the accompanying drawings illustrates a process of varying the extinction ratio with the EA modulation system. In FIG. 19, the horizontal axis represents a drive voltage and the vertical axis an optical output power. As shown in FIG. 20, an EA modulator 34 has its optical output power reduced as the drive voltage output from a driver 32 increases. A low input signal level is converted into a drive voltage level V0 and a high input signal level is converted into a drive voltage level V1 by the driver 32. These drive voltage levels are input to the EA modulator 34, which modulates DC light from a light source 30 with the drive voltage. Optical output power levels which correspond to the drive voltage levels V0, V1 at the time the optical output power is turned on and off are determined, thus determining an extinction ratio. In this manner, the extinction ratio is varied by controlling the drive voltage levels V0, V1.
A patent document indicated below discloses a system for controlling an external modulator to prevent the extinction ratio of an output optical signal from being degraded due to an operation drift independently of an input signal.
Patent Document:
Japanese Patent Laid-Open No. Hei 3-251815
The following methods are available for superposing noise.
FIG. 21 of the accompanying drawings illustrates a method of superposing noise. As shown in FIG. 21, for superposing noise, an electric main signal and an electric interference signal are mixed with each other by a mixer 42. The mixer 42 inputs the mixed signal to a driver 44. The driver 44 outputs a drive voltage, which comprises the main signal including the interference signal, to modulate DC light from a light source 40 with an external modulator 46.
FIG. 22 of the accompanying drawings shows another method of superposing noise. An optical interference wave shown in FIG. 22 is converted in level by a level converter 60, and then mixed with an optical main signal shown in FIG. 22 by a mixer 62. An optical output signal from the mixer 62 is converted in level by a level converter 64, which outputs an optical output signal shown in FIG. 22.
According to an aspect of the present invention, there is provided a light source device comprising a continuous wave (CW) light source for emitting first direct current (DC) light, a level converter for converting optical electric power of the first DC light into second DC light and outputting the second DC light, and a mixer for mixing an optical main signal having a constant extinction ratio with the second DC light. According to the method of superposing noise as shown in FIG. 21, it is difficult to design the mixer 42 because attention needs to be paid to electric connections in view of the impedance matching and RF characteristics of the interference signal and the main signal. According to the method of superposing noise as shown in FIG. 22, the maximum high level power is represented by the sum of the high level power of the main signal and the maximum power of the interference wave and the sum of the low level power of the main signal and the minimum power of the interference wave. Since the extinction ratio of the optical output varies depending on the magnitude of the interference wave, the magnitude of the interference wave cannot be changed while the extinction ratio is being constant.